More specifically, this invention relates to clamps as defined above, of the general type including a rigid frame, associated with a support such as a rigid foot or the robot, a mobile subassembly, associated with the frame and itself comprising a first so-called stationary arm, a second so-called mobile arm and a main actuator, capable of moving the mobile arm with respect to the stationary arm, according to a first degree of freedom, in translation or rotation, so as to close or open the clamp, in order, respectively, to grip a sheet metal assembly between the stationary and mobile arms (brought together by the actuator) or to release the sheet metal assembly (by separating the arms from one another by the actuator), which lamp also comprises a balancing module, introducing an additional degree of freedom between said support and an assembly integrating said mobile subassembly, so as to balance the forces exerted on the ends, respectively, of the stationary and mobile arms in the closed position, in which said balancing module comprises:                a device enabling a movement according to said additional degree of freedom, in translation, rotation or combination of the two, of said mobile subassembly with respect to the support, and        a balancing system including a flexible device, associated with said mobile subassembly and urging, according to said additional degree of freedom, at least one member associated with said support.        
In one application, in which the invention is of the greatest value for the applicant, the clamp is a clamp for welding by electrical resistance, in which case respectively the ends of the stationary and mobile arms are welding electrodes, respectively stationary and mobile. Similarly, the additional degree of freedom for performing the balancing operation can be a translation with a guide device, and the flexible device includes at least one balancing spring that extends parallel to the direction of movement in translation and, advantageously, consists of at least one pair of opposing springs working in compression.
On this type of clamp for welding by electrical resistance, belonging to the prior art, depending on the work to be performed, two kinematics are possible for the mobile arm and electrode:                the first degree of freedom is a translation of the mobile arm and electrode with respect to the stationary arm holding the stationary electrode, by linear guidance provided by the main actuator, which is a linear actuator of any suitable type known, hydraulic, pneumatic, mechanical, electric, which directly moves the mobile arm and electrode, in which the clamp is then said to be C- or J-clamps, as shown in the appended FIGS. 1 to 3, or        the first degree of freedom is a rotation of the mobile arm and electrode, around a pin on the frame, with respect to the stationary arm holding the stationary electrode, in which the clamps are then said to be X- or scissor-clamps, and in which the welding actuator can be a linear actuator, of the cylinder type, mounted either pivotally by its body on the stationary arm support around a pin parallel to the pivot pin of the mobile arm and electrode, or attached rigidly to this same stationary arm support, in which a suitable mechanical connection with two degrees of freedom then enables the rod of the cylinder moving linearly in a given direction to follow the pivoting movements of the mobile arm around the pivot pin. The pivot force of the mobile arm and electrode is transmitted from the actuator to the mobile arm by a lever, to which the mobile arm is secured, pivoting about the pivot pin, and on which the end of the rod of the actuator pivots, as shown in the appended FIG. 4.        
In FIGS. 1 to 4, the same references designate identical or equivalent components in the C- and X-clamps, shown in the various positions described below.
To ensure the electric spot welding of a sheet metal assembly 1, the C-clamp of FIGS. 1 to 3 includes primarily a stationary electrode 2 mounted at the end of a stationary arm 3 secured to a support of the stationary arm 3 and/or the body 5 of a main or welding actuator 4, for example of the pneumatic cylinder type, of which the piston 6 and the rod 7 are secured in movement with a mobile arm 8, guided by the support 5 and in the extension of the rod 7, and of which the free end supports a mobile electrode 9, in which said components form a subassembly mounted so that it is mobile, according to an additional degree of freedom that, in these figures, also corresponds to a translation, on a frame 10 rigidly attached to a support, which can be a manipulating robot or a rigid foot, in which said additional degree of freedom of the subassembly (2-9) with respect to the frame 10 is obtained by a module 11 described below.
Similarly, in the X-clamp of FIG. 4, the stationary electrode 2 and the stationary arm 3 are secured to a rigid arm support 12 mounted on a pivot pin 13 around which a rigid lever 14 supporting the mobile arm 8 and the mobile electrode 9 pivots, in which the welding actuator 4, also linear, pivots by its body 5 on the support 12 around a pin 15 parallel to pin 13, while the free end of the rod 7 of the actuator 4 actuates the lever 14, on which the rod 7 pivots around a pin 16 also parallel to pin 13, so as to control the pivoting of the mobile arm 8 and electrode 9 with respect to the stationary arm 3 and electrode 2 by rotation around the pin 13 supported by a rigid console 17 of the frame 10 of the tool, rigidly attached to the tool support (stationary foot or manipulating robot).
An additional degree of freedom, which is a rotation, is given to the subassembly of the stationary and mobile arms (3, 8) and electrodes (2, 9) and the welding actuator 4 with respect to the frame 10 by pivoting of said subassembly with the arm support 12 and the lever 14 around the pin 13 owing to a module 21, of which the structure and the functions are described below.
When the welding assembly process, which can begin once the sheet metal 1 is gripped between the electrodes 2 and 9, is automated, the clamp or the sheet metal assembly 1 to be welded is carried out the end of a pivotally connected arm of a manipulating robot. However, in the most common realization of the welding process, as described below, the clamp is transported by the robot and positioned in front of the sheet metal assembly 1, with the problem being the same in the reverse case in which the sheet metal assembly t is transported and positioned in front of the clamp.
According to the path programming, the robot positions the stationary electrode 2 of the clamp in front of the sheet metal assembly 1 to be welded. However, for technical reasons associated with:                the robot positioning precision;        uncertainty regarding the geometry of the clamp (tolerance for machining and assembly of components, possible wear of electrodes 2 and 9 in the process); and        the error concerning the actual position of the sheet metal assembly 1 with respect to its theoretical position,it is necessary, when programming the robot, to provide a certain distance between the theoretical position of the stationary electrode 2 at the end of the stationary arm 3 and the theoretical position of the sheet metal assembly 1.        
This distance must make it possible, during dynamic robot movement phases, to guarantee any absence of contact between the electrode 2 of the stationary arm 3 and the sheet metal assembly 1, so as to prevent any friction and/or contact of said electrode 2 on the sheet metal 1, causing marks, scratches or deformations of the sheet metal 1.
In practice, in the case of a resistance welding process, the value of this distance is between around 5 mm and around 15 mm and is called the relief course.
After this phase of positioning the clamp with respect to the sheet metal assembly 1, the additional degree of freedom mentioned above is released on the clamp, so that the mobile subassembly integrating the stationary and mobile electrodes and arms (2, 3, 8 and 9) and the welding actuator 4 can then perform a relative movement with respect to its frame 10, which relative movement, allowed by the aforementioned module 11 or 21, can be a translation, parallel to that or the mobile arm 8, as in the C-clamp of FIGS. 1 to 3, or a rotation around pin 13, as in the X-clamp of FIG. 4, which operation, called a docking operation, is intended to enable the electrode 2 of the stationary arm 3 to come into contact with the sheet metal assembly 1 to be welded.
Ideally, this docking movement must be performed fully, without creating any force or shock capable of deforming the sheet metal 1, even though said movement must be performed without knowing precisely the difference between the theoretical and actual positions of the sheet metal assembly 1 and the stationary electrode 2, for different masses of the clamp and different positions of its center of gravity, and regardless of the tilt of the clamp in space.
After or simultaneously to this docking operation, the welding actuator 4 is controlled and moves the mobile arm 8 so that the mobile electrode 9 closes in toward the stationary electrode 2 by gripping, between the two electrodes 2 and 9, the sheet metal assembly 1. After the mobile electrode 9 comes into contact with the sheet metal 1, a phase of applying and increasing the welding force takes place. However, in an empty closing of the clamp (in the absence of sheet metal 1), the position of the point of contact between the stationary 2 and mobile 9 electrodes drifts according to the difference in flexibility between the stationary arm 3 and the mobile arm 8, with the magnitude of this drift or movement being directly proportional to the value of the force applied.
Consequently, to prevent any deformation of the sheet metal 1, it is necessary for the position of the clamp (its electrodes 2 and 9) to be corrected throughout the phase of increasing the force, so that the point of contact between the electrodes 2 and 9 constantly corresponds to the actual position of the sheet metal assembly 1, with this operation being called the operation of balancing or centering the clamp on the sheet metal assembly 1.
As for the docking operation, for a clamp with a defined mass, this balancing operation must ideally be capable of being performed independently and with the same quality for all tilts of the clamp in space.
In general, when the natural point of contact between electrodes 2 and 9 is moved to the side of the stationary arm 3, we refer to under-docking, and, conversely, when this natural point of contact is moved to the side of the mobile arm 8, we refer to over-docking.
The consequences of a docking operation and/or a balancing operation improperly or not performed are a risk of producing a deformation in the sheet metal 1, which would be irreversible due to exceeding an elastic limit constraint of this same sheet metal once the welding assembly has been performed, and an imbalance in the force between the two electrodes 2 and 9 (loss of force on the stationary akin 3 in the case of over-docking, increase in force in the case of under-docking), which is detrimental to the quality of the assembly process. Indeed, the magnitude of the deformation of the sheet metal 1 with respect to the imbalance in force between the two electrodes 2 and 9 is a function of the rigidity of the assembly and the position of the gripping means.
Finally, after the docking and balancing operations, the welding operation can be performed with the creation of the assembly point. After this, it is necessary to perform an operation of returning and holding the clamp in the reference position, called the relief operation. In the so-called relief operation, the two electrodes 2 and 9 are moved away from the sheet metal assembly 13 to an initial position from which the docking operation, for the next welding point, can be controlled. Ideally, the balancing operation must be capable of being performed for all orientations of the clamp in space and preferably independently without any particular adjustment.
As indicated above, in applications in which the clamp is attached to a support and the sheet metal assembly 1 to be welded is transported by a manipulating robot, the problem remains the same, still requiring operations of docking and balancing the clamp on the sheet metal assembly 1, then of relief (return to the reference position), only the tilt of the clamp, in this case, is no longer a variable.
In the resistance welding clamps of the prior art and of the type indicated to above, according to FIGS. 1 to 4, the docking, balancing and relief operations are made possible by the additional degree of freedom, by translation of the mobile subassembly (2, 3, 4, 8, 9) with respect to the frame 10 on the C-clamps, or by pivoting (rotation) of the mobile subassembly around a pin 13 or the frame 10 on the X-clamps, and are ensured owing to module 11 or 21, respectively in the C- or X-clamps, essentially including at least one cassette 11a or 21a for guiding in translation and resilient balance by springs, and at least one actuator 11b or 21b for docking, relief and locking and holding the mobile subassembly in the relief position, which actuator 11b or 24b can have, when the guide cassette does not have a balancing spring, at least two resilient balancing means opposing and urging at least one member of said actuator that is secured in movement to said cassette, for example a pneumatic actuator with two pressurized gas chambers on each side of a piston rigidly connected by the rod of the actuator to a slide mounted so as to slide in translation into the cassette, so as to produce two opposing resilient balancing means.
In the balancing modules 11 of FIGS. 1 to 3 and 21 of FIG. 4, the docking and relief functions are associated with the balancing function, and the means for implementing these three functions are combined, and enable either a linear movement of the mobile subassembly, also called a cart, including the two arms 3, 8, the two electrodes 2, 9 and the welding actuator 4, with respect to the frame 10 rigidly attached to the support (see FIGS. 1 to 3), or a rotation movement of the mobile subassembly with respect to the frame 10 (see FIG. 4).
In FIGS. 1 to 4, each of the balancing modules 11 and 21 include a linear guide cassette 11a or 21a, also performing the additional function of docking and balancing by opposing springs, and a linear relief actuator 11b or 11b. 
When the three functions combined are performed by pivoting (rotation) movements of the mobile subassembly with respect to the frame 10, the implementation means can be transposed from those mentioned above, and include at least one rotary relief actuator as well as at least one cassette for guiding in rotation and docking and balancing by at least two opposing torsion springs.
In FIGS. 1 to 3, the C-clamp of the prior art has its balancing, docking and relief module 11 consisting of a cassette 11a for guiding in translation, docking and balancing with springs, and a relief actuator 11b, which is a simple-effect actuator, generally pneumatic or hydraulic.
The cassette 11a comprises a rigid rectangular body 22, in each of the two large opposite sides of which is provided one, respectively, of two identical recesses, through each of which, one, respectively, of two identical guide columns 23 pass through longitudinally, which columns are spaced apart, parallel to one another and to the large sides of the body 22, attached to the body 22 by their two ends, and mounded so as to slide each into one, respectively, of two identical slides or tubular sleeves 24, parallel and secured to the frame 10, while the body 22 is rigidly attached under the body 5 of the welding actuator 4.
The two opposite axial end portions of each column 23, which extend from each side of the corresponding sleeve 24, are each surrounded by one, respectively, of two identical and opposing helical springs 25, coming into contact by an axial end with the body 22, at the corresponding end of the corresponding recess, and by the other axial end with the corresponding axial end of the corresponding sleeve 24. Thus, four balancing springs 25 come into contact with the sleeves 24 rigidly attached to the frame 10 in order to urge the body 22 of the cassette 11a, and therefore also the mobile subassembly rigidly connected thereto, axially on one side or the other, in order to balance the clamp, after the mobile subassembly (2, 3, 4, 8, 9) has been translated following the docking movement from the relief position (initial reference) of FIG. 1 to the balancing position of FIG. 2, by inverting the two positions of the fluidic control unit 26, interposed between the actuator 11b and a pressurized fluid supply line and a fluid return line.
Initially, the mobile subassembly has been brought to the relief position (FIG. 1), in which the stationary electrode 2 is separated from the sheet metal 1, by admission of pressurized fluid into the working chamber 27a delimited in the cylinder 27 of the actuator 11b by the piston 28, which is secured by the rod 29 of the cassette 11a body 22, until the piston 28 abuts the cylinder 27. This movement of the piston 28 and of the rod 29 causes the translation of the entire cassette 11a body 22 with the mobile subassembly (2, 3, 4, 8, 9) in the direction that compresses the springs 25 to the right of the sleeves 24 and relaxes the springs 25 to the left of the sleeves 24. The mobile subassembly is then rigidly attached to the frame 10 and the clamp is in the so-called relief, or reference position. The inversion of the distributor 26 controls the docking of the stationary electrode 2 against the sheet metal 1 by emptying of the working chamber 27a previously pressurized in the actuator 11b, due to the relaxation of the previously compressed springs 25, driving the translation in the opposite direction of the cassette 11a body 22 with the mobile subassembly (2, 3, 4, 8, 9), thus released from its stop, until the mobile subassembly and the cassette 11a body 22 are balanced between the two pairs of opposing springs 25 (see FIGS. 2 and 3).
The stroke of the docking operation is dependent on the stiffness and the tension of the springs 25, the mass and the tilt of the load to be balanced and possible friction that may slow the movement of the load. When the balancing position has been achieved, the mobile subassembly retains a certain axial flexibility, owing to the springs 25.
In this example, the cylinder 27 of the actuator 11b is rigidly attached, like the sleeves 24, to the frame 10, and the body 22 of the cassette 11a is rigidly attached to the mobile subassembly. Alternatively, the sleeves 24 can be secured to the mobile subassembly, and the cassette body 22 can be secured to the frame 10, in which case the rod 29 of the piston 28 of the actuator 11b drives the sleeves 24 passing through the body 22. Also alternatively, the cylinder 27 or the actuator 11b can be secured to that of the element(s), among the body 22 and the sleeves 24, which is (are) secured to the mobile subassembly, in which case the rod 29 of the piston 28 comes into contact with the other element(s) mentioned above, which is (are) secured to the frame 10, in order to move the mobile subassembly in translation with respect to the frame 10, against the action or under the action of the springs 25.
In these variants, as in the clamp of FIGS. 1 to 3, the mobile subassembly retains a certain axial flexibility, owing to the springs 25.
However, these realizations face a contradiction between, on the one hand, the need to balance, over a short course, a large tool mass, regardless of the orientation of the clamp, resulting in the need to use springs with high stiffness, and, on the other hand, the need for maximum flexibility of these same springs 25 in auto-balancing operations of the clamp on the sheet metal 1.
In practice, these realizations are fairly unsatisfactory, difficult to implement and are applicable only if the mass of the clamp is limited, and/or for sheet metal assemblies 1 of high thickness (the sheet metal assembly 1 with good rigidity) and/or for limited changes in the orientation of the clamp with respect to a vertical axis.
In a variant of the C-clamp, the cassette differs from that 11a of FIGS. 1 to 3 only by the absence of springs 25, so that it satisfies not the function of docking and resilient balancing, but only the function of guiding the mobile subassembly in translation with respect to the frame 10 owing to the columns 23 and the sleeve-slides 24. The docking and balancing function is then performed by the relief actuator 11b, which is a double-effect pneumatic cylinder, in which the cylinder 27 is secured to the frame 10 and the rod 29 of the piston 28 is secured to the cassette body 22, itself secured to the body 5 of the welding actuator 4. The pneumatic control is provided by the two-position distributor 26 cooperating with a control valve (not shown) enabling the control or blocking of the supply or emptying of compressed air to or from one 27a of the two working chambers of the actuator 11b, which compressed air is emptied from the other working chamber or supplied to this other chamber through the distributor 26.
In a first phase, the chamber 27a of the cylinder 11b is supplied, while the other chamber is emptied through the distributor 26 so as to push the piston 28 so that it abuts the cylinder 27, and, therefore, by the rod 29, the mobile subassembly in the relief position (reference position abutting the frame 10) so that said mobile subassembly is then rigidly connected to the frame 10, with the stationary electrode 2 being separated from the sheet metal 1. The control valve makes it possible to isolate the pneumatic actuator 11b in this position, in order to hold the clamp in this relief position.
Then, the docking and balancing operations are performed by controlling the inversion of the distributor 26 so as to re-pressurize the other chamber and by controlling, by means of the valve, the pressure difference between the two chambers of the cylinder 11b, so as to move the piston 28 and the rod 29, and therefore the mobile subassembly, so as to bring the stationary electrode 2 in contact with the sheet metal 1 (docking) and to compensate for the effect of the mass of the load (balancing).
Owing to the control of the pressure difference between the chambers of the cylinder, and the surface area difference between the two opposite faces of the piston 28 associated with the presence of the rod 29, the clamp is finely balanced over the entire stroke necessary for the docking operation. By using, as the distributor 26 and control valve, at least one proportional control regulator, the balancing operation can be performed for all of the orientations of the clamp in space as the welding process is implemented.
The docking, balancing and relief are therefore pneumatic in this variant of the C-clamp.
In this last variant, the two pressurized gas chambers on each side of the piston 28 rigidly connected by the rod 29 to the cassette body 22, and therefore to the mobile subassembly create the two resilient opposing balancing means.
However, in practice, this principle of pneumatic balancing docking and relief has the disadvantage of requiring pneumatic energy to be provided on the clamp. This is costly, very specifically when the main actuator 4 driving the mobile arm 8 is electric. Indeed in this case, the relied docking and balancing functions are the only functions requiring compressed air.
In addition, the numerous constraints of the process and associated with the products to be produced do not enable a single solution to be proposed that is compatible with the needs of different users. In conclusion, the pneumatic realizations, while capable of being high-performing, are not entirely satisfactory.
Similarly, the X-clamp of FIG. 4 enjoys a spring balance with the guide cassette 21a of the module 21, also performing the docking function, while the relief function is provided by the pneumatic or hydraulic single-effect cylinder-type actuator 21b, while, alternatively, it is possible for the balancing module 21 to include not a guide cassette 21a, but only a pneumatic double-effect cylinder-type actuator 21b, performing the functions of docking, balancing and relief of the clamp.
More specifically, in FIG. 4, the guide and balancing cassette 21a includes a piston 31 secured to a rod 32 mounted on the arm support 12 so as to pivot around a pin parallel to the rotation pin 13, and the piston 31 is mounted in a sliding way in a cylinder 33, which is pivotally mounted by its end opposite that through which the rod 32 passes, on the rigid console 17 of the frame 10, around a pin also parallel to the rotation pin 13, in which the cylinder 33 houses two helical and opposing springs 34 each in contact with one of the ends of the cylinder 33, on one side and on the other, against one, respectively, of the two opposite faces of the piston 31. Thus, the springs 34 provide the balance of the mobile subassembly in rotation around the pin 13 with respect to the frame 10, Linder the same conditions as the springs 25 of the C-clamp of FIGS. 1 to 3, and with the same disadvantages resulting from the need to have springs that are both flexible and significantly stiff, which is contradictory.
In the single-effect linear actuator 21b of the X-clamp of FIG. 4, the cylinder 35 and the rod 36 are pivotally mounted, by their opposite ends, respectively, on the rigid console 17 of the frame 10 and on the arm support 12, around pins parallel to pin 13, and, the piston 37 secured to the rod 36 delimits, in the cylinder 35, a working chamber 35a (on the side of the rod 36), which selectively communicates, by the same fluidic distributor 26 with two positions of FIGS. 1 to 3, with a pressurized fluid supply or a return line for said fluid, in which the supply of pressurized fluid to this working chamber 35a enables the piston 37 to be moved so as to abut the pivotally mounted portion of the cylinder 35, thereby causing the arm support 12, and therefore the mobile subassembly supported by the latter, to pivot in rotation around the pin 13 in order to bring and hold this mobile subassembly in the relief position (initial reference position) in which the stationary electrode 2 is separated from the sheet metal 1, while inside the cassette 21a, the piston 31 is drawn toward the outside of the cylinder 33 (on the side of the rod 32), thus compressing one of the springs 34 and relaxing the other. By controlling the inversion of the control distributor 26, the working chamber 35a of the actuator 21b is depressurized owing to the communication with an exhaust outlet 26a. Under the action of the balancing springs 34, the piston 31 returns to a balancing position inside the cassette 21a by causing the arm support 12 to pivot with respect to the rigid console 17 of the frame 10, which pivoting of the arm support 12 simultaneously exerts a pulling force on the rod 36 of the actuator 21b, of which the piston 37 is moved in the direction tending to reduce the volume of the working chamber 35a, and so that the stationary, electrode 2 at the end of the stationary arm 3 connected to the support arm 12 comes into contact with the sheet metal 1, in the docking and balancing position.
Alternatively, the pneumatic balancing of the clamp can be performed by the actuator 21b, which is then, as already mentioned, a double-effect pneumatic cylinder, controlled by the distributor 26 and a control valve (not shown) used under the same conditions to perform the pneumatic balancing of the pneumatic variant of the C-clamp mentioned above. Indeed, by controlling the distributor 26, the other chamber of the actuator 21b is connected to an exhaust outlet and therefore depressurized. The piston 37 then abuts the pivoting end of the cylinder 35, thereby enabling the clamp to be held in this relief position.
Then, the docking and balancing operations are performed by controlling the inversion of the distributor 26 so as to re-pressurize the other chamber and by controlling, owing to the valve, the pressure difference between the two working chambers of the cylinder, so as to compensate for the torque around the rotation pin 13 created by the mass and the position of the center of gravity of the mobile subassembly, and, as in the aforementioned variant of the C-clamp, at least one proportional control regulator is preferably used so that the balancing operation can be performed with the same flexibility for all of the orientations of the clamp in space, during the welding process.
However, this X-clamp variant with pneumatic relief, docking and balancing has the same disadvantages as the analogous pneumatic C-clamp variant, associated with the need to provide pneumatic energy to the tool.
Another solution consists of entirely eliminating, on the clamp, the degree of freedom associated with this balancing function. Indeed, it is possible to envisage a robot being informed, with sufficient precision, about the change in geometry of the tool, associated with the wear of the electrodes and with the drift of the point of contact between the electrodes when applying stress.
By knowing these values, the uncertainty on the relative positioning of the sheet metal 1 is reduced, and the robot can itself perform the docking, and then the balancing operation of the clamp on the sheet metal assembly 1.
This solution has the benefit of being economical, owing to the elimination of the degree of freedom on the clamp body, and of being operational independently of the orientation of the clamp in space.
However, this solution involves:                knowing precisely, and owing to regular tests, the degree of wear of the stationary electrode 2,        knowing the drift of the point of contact between the electrodes 2 and 9, when applying the load,        using a feedback controlled cylinder 4 (generally electric) enabling the deformation of the clamp to be monitored when the force is increased, and        using a robot specifically programmed to enable the clamp position to be corrected in a manner synchronized with the wear and deformation of the clamp.        
The balancing operation is not flexible and can be performed only by using, as a set point, the theoretical position of the sheet metal 1, as error of positioning thereof cannot be taken into account, with the consequence of either a deformation of the sheet metal 1, or an imbalance in force, detrimentally affecting the quality of the welding process.
Finally, the times necessary for the robot to correct its path are relatively long, and lead to a cycle time loss with respect to the traditional realizations mentioned above.